This invention pertains in general to the detection of impact on a wall by objects such as projectiles, debris or loose parts in an environment surrounded by a wall, and more particularly to the discrimination between an electrical signal outputted by a transducer upon impact and parasitic signals which could be taken for an impact representative signal.
It is desirable in the operation of pressurized water reactor power generating stations to provide a system which will enable an early detection and survey of the failure of primary system components. Early detection of component failures will prevent the occurrence of dangerous operating conditions such as failure of mechanical components which characteristically results in metal debris which concentrate in the steam generator inlet plenum and the bottom plenum of the reactor vessel. Those locations being the most probable collecting points are the most suitable for the detection by a monitoring system. The debris are transported to those collecting points by the normal flow of the primary coolant and are propelled during the course of travel against the walls enclosing the primary system coolant paths. Accordingly, transducers judicially placed at different locations against the walls will provide an indication of any such primary system component failure.
A system has been employed in the past for detecting the mechanical state of a machine which is described in U.S. Pat. No. 3,554,012, issued Jan. 12, 1971. The system there described converts the mechanical vibrations occurring within the machine into analogous electrical vibrations by means of a sensor, and analyzes the output electrical oscillations. During the conversion, initial shock waves radiate from the point of impact and create mechanical transients in the measuring system. This technique utilizes the transient response at the resonant frequency of the transducer, rather than the initial shock response of the transducer to obtain the output parameters. The resultant response is a continuous oscillation of relatively low amplitude at the resonant frequency of the transducer which includes transients within such continuous oscillation. The equipment is designed to discriminate between relatively small amplitude levels, thereby to lower the sensitivity of the system response.
An improvement over this is described in U.S. Pat. No. 3,860,481, issued Jan. 14, 1975. The concept, here, is to generate discrete outputs indicative one of the rate of the energy impact, the other of the impact energy.
While piezoelectric accelerometers which are used to detect signals generated by metallic impacts within nuclear reactor vessels provide an amplified output signal (frequency range 0-20 kHz), the impact generated signals must be distinguishable from both the flow generated background noise and the electrical noise "spikes".
The simplest known method for such discrimination consists in comparing the transducer output signal with a fixed setpoint. If the signal level exceeds the setpoint an impact is said to have occurred.
The shortcomings of this method are twofold. First, the background noise level varies as plant operating conditions change. Since the setpoint must be higher than the highest background noise level ever present, an unnecessarily high setpoint exists when the background noise level is low. Secondly, the device makes no attempt to distinguish electrical noise spikes from impact signals.
Another approach stems from the fact that hard object impact signals are of greater duration than electrical noise spikes. The transducer output signal is put through an envelope detector. Discrimination against background noise is then accomplished on a "level setpoint" basis, whereas discrimination against noise "spikes" is on a "time above setpoint" basis. Although this method can be used with success in general, the envelope may be too idealized a waveform. In practice, the envelope may be ineffective to detect an actual impact.